Convert CONFIG_HOSTNAME et al to Kconfig

This converts the following to Kconfig:
   CONFIG_GATEWAYIP
   CONFIG_HOSTNAME
   CONFIG_IPADDR
   CONFIG_NETMASK
   CONFIG_ROOTPATH
   CONFIG_SERVERIP
   CONFIG_UBOOTPATH

To do this, we introduce a CONFIG_USE_ form of each of the above and
change include/env_default.h to test for that to be set before setting a
value. Further, we don't want to stringify the IP address related values
as they are now properly strings via Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/seeed_npi_imx6ull_defconfig b/configs/seeed_npi_imx6ull_defconfig
index 34d23fd..abcd63c 100644
--- a/configs/seeed_npi_imx6ull_defconfig
+++ b/configs/seeed_npi_imx6ull_defconfig
@@ -49,6 +49,8 @@
 CONFIG_USE_ETHPRIME=y
 CONFIG_ETHPRIME="eth0"
 CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_USE_NETMASK=y
+CONFIG_NETMASK="255.255.255.0"
 CONFIG_FSL_USDHC=y
 CONFIG_MTD=y
 CONFIG_DM_MTD=y